Process of removing impurities from fresh spun polyacrylonitrile gel filaments by directing inert gas bubbles against the filaments



Aprll 30,v 1957 T. H. ROBERTSON ETAL 2,790,698

PROCESS OF REMOVING IMPURITIES FROM FRESH SPUN POLYACRYLONITRILE GEL FILAMENTS BY DIRECTING INERT GAS BUBBLES AGAINST THE FILAMENTS Filed Dec. 9, 1953 v (a M United States Patent O PROCESS F REMOVING IMPURITIES FROM FRESH SPUN POLYACRYLONITRILE GEL FILAMENTS BY DIRECTING INERT GAS BUBBLES AGAINST THE FILAMENTS Thomas H. Robertson, Stamford, and Percival W. Curnmings, Jr., Cos Cob, Conn., assignors to American Cyanamid Company, New York, N. Y., a corporation of Maine Application December 9, 1953, Serial No. 397,161

7 Claims. (Cl. 8-137.5)

This invention relates to an improved continuous process for extracting soluble material from a permeable article moving through a liquid bath.

. Continuous countercurrent extraction methods have long been employed in the manufacture of various artificial and synthetic bers by wet yspinning processes. In these the coagulated filaments have been passed through one or more baths containing a solvent miscible with the solvent employed in the spinning solution or in the coagulating bath in order to free the coagulated article from the residual spinning solvent or coagulant.

An increase in the eiiiciency of these extractions is highly desirable in order to reduce the quantity of Wash liquid required, the power required for pum-ping such liquid and the size of the extraction tanks. Where it is possible to recover the spinning solvent or coagulating liquid, an improved extraction process is doubly desirable in reducing the cost of the concentration or recovery of such materials from the wash liquid and in minimizing the quantity of solvent or coagulant required for make-up.

An object of the invention is to provide an improved method for extracting substances from a shaped permeable article containing extractables.

Another object of the invention is to provide an improved method of extracting liquids from gelatinous films or filaments produced by the Wet spinning of a solution of polymeric material.

Other objects of the invention will be apparent to those skilled in the art from th-e detailed description hereinbelow.

It has been discovered that extraction eiiiciency is remarkably improved by directing the bubbles of an inert gas against the surface of a shaped permeable article moving continuously through a liquid extraction medium;Y

It is diihcult to obtain thorough extraction of processing material or other foreign substances from a continuous shaped gelatinous or other permeable article Without using excessive amounts yof extractant and thereby producing an extract so dilute that the cost of recovering'the foreign substances is high, and sometimes not economically justifiable. Nevertheless, -substantially complete removal of processing materials from the gel is often necessary to obtain certainV required physical characteristics or propertiesin the` final article. When the obvious expedient ofv increasing the volume of Wash liquid is employed to reduce the concentration of foreign matter in the nal article, the extraction process becomes distinctly lessrfavorable from an economic standpoint. Various expedients were tried with little or no success to reduce the `necessary volume of wash water while maintaining the same degree of extraction used in extracting spinning solventy from a polymeric acrylonitrile tow or strand of filaments in gel form intended for the manufacture of Patented Apr. 30, 1957 ICC staple fiber. In one of these experiments, two of a series of six wash troughs were mounted on exible supports and the troughs were shaken bodily by means of motordriven eccentric drives while the tow was passing therethrough. In another trial,I the wash liquor in one of the baths was recirculated to the same bath by means of a pump at a 140% recirculation rate (1.4 gallons recircu-l lation per a gallon of fresh liquid introduced into the tank) with the liquid reintroduced through a fan-shaped distributor having a lg inch slotted orifice directing the liquid against the tow strands from beneath in orde-r to.

agitate them. The minorl improvement in extraction efficiency was found to be insutlicient to justify the pumping costs.

In View of the failure of simple agitation and a liquid jet to achieve the desired result, it is surprising that it can be accomplished by a mere stream ofr bubbles of air or other inert gas. The reasons why the stream of bubbles produces such striking effects are not fully known, but'it is thought that the bubbles bring more or fresher extracting solvent into contact with the material to be extracted. `However, it is apparent that the process is a physical rather than a chemical one since there are no temperature changes or differences in the final product which suggest that a chemical reaction has taken place.

The process described hereinbelow is applicable to the' extraction by means vof a suitable solvent or extractant liquid of any soluble materialin either solid or liquid form from a continuous or substantially endless multi-g or orifices distributed along the bottom of the wash tanki .ln another variation -the air may be bubbled through porous discs or plates of any suitable materials, including refractories and porous powdered metal structures, instead of simple holes or orifices.

channels.

Any gas may be employed in the present process provided that it is inert, that it does lnot react with either ,thef shaped article or the extraction medium and further isl no completely soluble in the wash liquid at the tempera-Q ture used. In general, it is recommended that a gas be` selected which is not soluble to .more than a small extentv` in the liquid medium. Air is, of course, preferred for any extraction operation in which it lits these requirements because of its low cost and ready availability.

For a better understanding of the nature and objectsv of this invention, reference should be had to the accom-` panying drawing in which the figure is a perspective view 4 of an extraction operation according to the` present invention.

The wash trough 2 has inside dimensions of approximately 12.5 feet in length and l5 inches in width. The"f aqueous extraction solvent for bath 4 is introduced'` through pipe 6 and overows through outlet pipe 8 which is positioned to provide a bath about 9 inches deep. 'In this bath are located the rolls 1i) and 12 which, are mounted in suitable journals of known construction (not. shown) and which are desirably positively driven by suit- For example, the entire wash tank may be provided with a false bottom constructed `of one of these porous materials to provideV streams of bubbles through the entire area of the tank; or porous discs or strips may be inserted in pipes or other'.

able conventional means. ln addition air under slight superannospheric pressure is supplied through. a vertical line 14 to a horizontal distributor pipe 16 which is disposed across the width of the bath with its upper face located approximately4 to 6 inches below the surface of bath 4. The upper face of pipe 16 is perforated with 'a plurality of `orifices 18, for example 3.6 in a distance of l2 inches, of relatively small diameter as, for example, onesixteenth of an inch or one-thirty-second of an inch, or even smaller. One or more multifilamentstrands 20 of an acrylonitrile copolymer fiber in the coagulated swollen gelatinous state passes downward into the bath under roll 10 and then through the length of the bath to roll'12 around which it passes before emerging from the bath for further processing elsewhere. While the strands 20 are passing through the bath air or another inert gas is being emitted from the orifices 18 in pipe 16 under slightly superatmospheric pressure to form streams of small bubbles. The fine bubbles of gas rising through the extraction bath are thus directed against the strands 20 to produce the effects described herein.

Excellent extraction results have been obtained with rolls 10 and 12 so located to position the strands 20 about 1.5 to 2 inches beneath and parallel to the surface of the bath 4. Accordingly, orifices 18 are about 2 to 4 inches below the strands. Since there is reason is believe that relatively small bubbles, ranging from those barely visible upward in size to about 0.3 inch diameter, provide greater extraction efficiency than is realized with larger bubbles; the holes 18 should not be located unnecessarily far below the strandsv 20. There is a tendency for the bubbles to coalesce into larger bubbles as they rise through the liquid, hence the greater the ydistance the bubbles travel the greater the opportunity fo'r them to form larger bubbles. In general it is recommended that the orifices should be located between about 2 and between about 6 inches below the gelatinous article which is being subjected to extraction, the optimum for each particular extraction can be readily determined by experiment. Where desired more than one bubbling device 16 may be 1ocated in each extraction tank to further increase the extraction efiiciency in that tank. ln a 6-stage extraction operation using 6 tanks similar to trough 2, it has been found that a gas bubbling device 16 will increase the extraction efficiency at any stage in such processes. It will be noted that the strands 20 move through the tank 2 in a direction countercurrent to the ow of the extraction medium. For maximum extraction eiciency, the gelatinous fiber to be washed should be passed through the tanks in sequential order while water or other fresh extraction medium is introduced first into the sixth tank, then into the fifth tank, and thereafter into the fourth tank, countercurrently to the movement of the gelatinous strands.

Although the process of the present invention is especially adapted for the extraction of multilament tow prepared from polymeric acrylonitrile, that is homopolymers or copolymers of acrylonitrile with compatible ethylenic monomers such as 2-methyl 5-vinyl pyridine, other vinyl pyridiues, methyl acrylate, vinyl acetate, vinyl chloride, etc., according to the processes described in U. S. Patents Nos. 2,558,730 to 2,558,735 inclusive; it is to be understood that the present extraction process is not limited to these particular polymers or to filamentary materials alone. In brief, the process of the aforesaid patents entails spinning a solution of polymeric acrylonitrile with a molecular weight of about g 75,000 (Staudinger) dissolved in concentrated aqueous sodium thiocypanateinto a relatively dilute 10% aqueous sodium thio'cyanate solution which is maintained at a temperature of approximately C. The vdistended gel filaments must be thoroughly extracted to remove substantially all of the thiocyanate solution in order to produce a fiber of satisfactory physical characteristics for textile uses. It is also contemplated that the'instant process is suitable for use in conjunction with any wet-spinning or wet-coagulation operation of a continuous nature as exemplitied by the manufacture of both` viscose and` cuprammonium rayon, the manufacture of cellophane and other lm or sheet materials as well as the wet-spinning of solutions of polyacrylonitrile in other known spinning solvents. The rate of bubbling a gas against the coagulated shaped article has been found to be signicantjin connection with extraction efficiency. In extracting solvent from tow prepared according to the process of the aforementioned patents with strands having 2330 filaments each of about 20 denier in the swollen gelatinous state (3 denier in the finished fiber), it appeared that air velocities as low as 0.1 foot per second at the orifice provide appreciable improvement over conventional extraction; however the air velocities above about 0.2 foot per second are recommended and the best results are obtained with orifice air velocities above about 1.0 foot per second. lt is thought that the optimum air rate will vary for different articles in the present process. For instance, it is to be expected that a film or a comparatively thick monolament Will each have a different optimum air rate than that of a multifilament strand. Also it is likely that the permeability or gel structure of the product and probably also the chemical composition will affect the optimum air rate. Accordingly, it seems undesirable to attempt to precisely define the over-all range of suitable air velocities except for the generality that the orifice air velocity may range from about 0.1 foot per second up to velocities approaching the point at which damage tothe article commences. In the case of the aforementioned polymeric acrylonitrile tow, the air rate was increased until the strands of tow were being lifted clear of the b ath without damage to the fibers. The optimum air velocities at the orifice can readily be determined in each individual case by simple experimentation; thus those skilled inthe art will have no difliculty in ascertaining the proper amount of air to be employed.

`The tremendous improvement `in extraction eiciency obtainable by the process of the present invention is evident from the results set forth in the table below which .sets forth results secured with apparatus of the type described herein in extracting sodium thiocyanate solution from a copolymer of acrylonitrile and 5% methyl acrylate prepared accordingto the process of the patents mentioned earlier. i

TABLE Extraction of sodium thocyanate solvent from polymeric acrylonitrle tow Run No 1 2 3 t Tow Speed, Meters/minute 81. 5 81. 5 81.6 0.5. 0 Production Rate, Pounds ber] hour 31. 5 31. 5 31. 5 36. 5 Wash Water Flow Rate:

Gallons/minute. 1. 8 1.8 1. 8 Gallons/pound ber 3. 38 3. 3B 2. 93 Air Bubbling Yes Yes Yes Estimated Air Rate, feet/second Gaz diameter oriflees) 0.5 0. 5 0. 5 5 Tow Strands Separated Yes Yes No N n Average Residual NaCN S on Tow Bone Dry Fiber Basis, percent:

'Iow Leaving- Coagulation Bath 155. 0 159. 3 V 143. 2 162. 0 Trough No. l 58.8 54.7 bT. 7 91.2 Trough No. 2 29. 3 30. 9 28. 5 44. 0 Trough No. 3 1G. 5 `0. 2 7. 6 14. t Trough No. 4-; 8. 5 1. 4 1. 8 2. 8 Trough No. 5 6. 0 0. 26 0. 39 0.55 Trough No. 6 4.0 No trace 0.10 0. 05

run 1 under otherwise similar conditions. It is also noteworthy that the 5 strands -of tow were not separated in runs 3 and 4, but were passed Ithrough the baths as a single broad band containing .about 11,650 individual lilaments to pose a more difficult extraction test. The comparison is even more striking in run 4 when it is noted that lthe speed yof .tow through the baths was .increased .more than without impairing the extraction efficiency even though the wash Water ow was maintained at the same total rate as in runs 2 and 3.

Since variations and modifications can be made in `the process of the present invention without departing from the .spirit of the invention, it is to be understood that the specilic details `and data set forth above are intended purely for the purpose of illustration and not to be construed in a limiting sense.

We claim:

il. A process which comprises directing bubbles composed essentially of an inert gas against the surface of a continuous fresh .spun gel lament comprising a polymeric acrylonitrile impregnated with an aqueous salt solution of a spinning solvent lfor said polymeric acrylonitrile until .some of said aqueous salt solution is removed =theretrom by a physical action produced by the inert gas bubbles and while the filament is moving continuously through and beneath the surface `of .a liquid solvent which is inert and nonsolvent for the polymeric acrylonitrile, and is a solvent for the said aqueous salt solution.

2. A process which comprises directing bubbles cornposed essentially of an iner-t gas against a strand of .a plurality of continuous fresh spun gel filaments comprising a polymeric acrylonitrile impregnated with an aqueous salt solution of a `spinning solvent for .said polymeric .acrylonitrile -until some tof said .aqueous salt solution is removed therefrom by a physical action produced by the inert gas bubbles and while the strand is moving continuously through and beneath 4the surface of a liquid solvent which is inert and nonsolvent for the polymeric acrylonitrile, and is a solvent for the said aqueous salt solution.

3. A process which comprises directing bubbles composed essentially of :against Ia strand yot" a plurality of continuous fresh spun vgel iilaments comprising a polymeric acrylonitrile impregnated with an aqueous salt solution of a spinning solvent ttor said polymeric acryl-onitrile until some of said aqueous salt solution is removed therefrom by .a physical action produced by the lair bubbles and while the strand is moving continuously through and lbeneath the surface of a liquid solvent which is inert and nonsolvent for the polymeric acrylonitrile, and is a solvent -for the said aqueous salt solution.

4. A process which comprises directing bubbles composed essentially of an inert gas against a strand of a plurality of continuous fresh spun gel filaments comprising a polymeric acrylonitrile impregnated with an aqueous sodium fthiocyana-te solution Iuntil some of said sodium thiocyanate solution is removed therefrom by a physical `acti-on produced by the inert 4gas bubles .and While said strand is moving continuously through and beneath 'the surface of an aqueous medium which is inert and nonsolvent for the polymeric acrylonitrile, and which is .a solvent for 'the lsodium thiocyanate solution.

5. A process which comprises directing bubbles corn- .posed essentially of air against a strand of a plurality of continuous fresh spun gel ilaments comprising a polymerio acrylonitrile impregnated with an aqueous sodium thiocyanate solution until some -of said ywdivm thiocyanate solution is removed therefrom by a physical action produced by the air bubbles and while said strand is moving continuously through and beneath `the surface of an aqueous medium which is inert and nonsolvent for the polymeric acrylonitrile, and which is a solvent for the sodium thiocyanate solution.

6. A `process according to claim 5 in which the extraction operation is repeated by immersion ot the strand of `gel filaments in .a Iseries :of baths with the aqueous medium and the strand being passed countercurrentl y from bath to bath.

7. A `process according to claim 5 characterized by the bubbles of air being introduced into the aqueous medium from an orice at an `orice velocity of at least 0.1

feet per second.

References Cited inthe file of this patent UNITED STATES PATENTS 1,915,072 Richter et al. June 20, 1933 1,948,568 Faber et al. Feb. 27, 1934 2,023,013 Faber et al. Dec. 3, 1935 2,394,957 White Feb, 12, 1946 2,431,956 Moody Dec. 2, 1947 OTHER REFERENCES Journal of the Textile Institute, December 1944, page A506. 

1. A PROCESS WHICH COMPRISES DIRECTING BUBBLES COMPOSED ESSENTIALLY OF AN INERT GAS AGAINST THE SURFACE OF A CONTINUOUS FRESH SPUN GEL FILAMENT COMPRISING A POLYMERIC ACRYLONITRILE IMPREGNATED WITH AN AQUROUS SALT SOLUTION OF A SPINNING SOLVENT FOR SAID POLYMERIC ACRYLONITRILE UNTIL SOME OF SAID AQUEOUS SALT SOLUTION IS RE- 